Electrical impedance tomography (EIT) is a non-invasive imaging technique used to investigate and measure regional lung ventilation and perfusion (flow of blood) in humans and animals. In contrast to conventional methods, EIT does not require the patient to breathe through a tube or sensor, does not apply ionizing X-rays and can be used for extended periods, say 24 hours or even longer. EIT can be used continuously and is therefore suited for monitoring treatment effects in real time and over time, EIT was first used to monitor respiratory function in 1983 and remains the only bedside method that allows continuous, non-invasive measurements of regional changes in lung volume, blood flow, and cardiac activity. More details of this technique can be found in “Electrical impedance tomography” by Costa E. L., Lima R. G, and Amato M. B. in Curr Opin Crit Care, February 2009, 15(1), p. 18-24.
Traditionally electrodes used for electro impedance measurements and in particular electro impedance tomography (EIT) are placed individually on the thoracic surface to form an electrode plane. Such individual electrodes are cumbersome to handle and are bound to be dislocated with patient movement. In order to overcome the above problem, belts (or belt-like structures) have been designed on which electrodes are mounted. The conventional belt is placed around the chest thereby forming a transverse EIT plane. For example document US 2009/0084674 A1 discloses an electrode assembly for electrical impedance tomography comprising a plurality of different electrode modules. Each module contains a support strap carrying a predetermined number of electrodes with a distance between each two consecutive electrodes predetermined as a function of a specific operational pattern. Also here the belt is placed around the chest, thereby forming a transverse EIT plane.
While such conventional belts solve the problem of electrode handling, despite the fact that they are often elastic, they restrict the breathing of the patient particularly by preventing the ribs from lifting upwards during respiration. Conventionally the belt is placed at an angle with respect to the extension of the ribs around the chest. As this angle is bigger than zero, there is a force preventing the free movement of the ribs. Consequently, to some extent the natural movement of the chest during respiration is restricted.
Such external restriction of the chest movement during respiration however can prevent optimal ventilation of a patient or produce artifacts during EIT examination.
In EIT, as disclosed by U.S. Pat. No. 5,626,146, a plurality of electrodes, typically 8 to 32, are arranged on the surface of the body to be examined. A control unit ensures that an electrical signal, for example a current is applied to one or several pairs of electrodes on the skin to establish an electrical field, which in turn is measured by the other electrodes.
Typically the electrodes are arranged on the surface of the body to be examined in such a way that they surround the body girth. Conveniently a belt structure supports the electrodes and is located about the chest of the patient in a transverse plane. Such positions have been shown e.g. in patent application EP 1 000 580 A1.
A good contact of the electrodes with the body of a test subject is essential to the EIT method. In order to establish good contact electrode belts have been proposed which are elastic, see e.g. patent specification GB 2 400 915 B. An elastic belt is under a certain pretension, as a result of which a radial force component acts as a pressing force on the electrodes and also on the test subject's body. To a certain extent such forces impede normal chest movement and extension during breathing. Due to the elasticity of the belt special provisions for electrode supply lines need to be made, such as folding supply lines in zigzag or meandering manner, to compensate for expansion of the belt material Moreover, in order to ensure further improved electrode contact also at body areas with inward curvatures, air-tight elements filled with air, liquid or gel are placed at the back of the electrodes.
Patent application US 2004/0236202 A1 proposes an alternative expandable electrode belt made of material that is flexible and bendable, yet inelastic. Hereby waves or bends in the belt allow the belt to be extended, while the electrodes remain spaced apart equidistantly as the strap extends—either during application onto or respiration by a patient. Physically, such belt will buckle as it is extended, which is disadvantageous where the test subject is a patient lying in bed.
Other belt-type garments are designed for the purpose of carrying sensors. For example, patent application US 2008/0287770 A1 discloses a garment for ambulatory, physiological monitoring of a patient comprising a belt, having first and second end portion with closures at the end portions to wrap around a user's chest, a pair of shoulder strap portions, and a back portion, with at least one of the belt portions, the strap portions and the back portion having an accommodation for carrying one or more singular sensors, such as an ECG sensor. Another example is given in patent specification U.S. Pat. No. 5,353,793 A. Here is disclosed another harness-like garment enabling measurements of physiological parameters of a patient. This garment comprises a chest band, an optional shoulder band(s), and an optional abdominal band that can house respiration, pulse and ECG sensors for impedance pneumography. The garment is stretchable and. all bands are worn on the body under tension. In a presented example several electrodes are arranged on the front of the shoulder band(s) and at some parts of the chest hand. The chest band apparently forms a transverse sensor plane.